This invention relates to an enhanced constellation display for VSB television signals.
G. Sgrignoli, W. Bretl and R. Citta, xe2x80x9cVSB Modulation Used for Terrestrial and Cable Broadcasts,xe2x80x9d ICCE95GS.DOC, June 1995, pages 1-16, the disclosure of which is hereby incorporated by reference describes a vestigial sideband (VSB) digital transmission system for terrestrial and cable television broadcasting. One implementation of this VSB digital transmission system is referred to as a trellis-coded 8VSB system. In the trellis-coded 8VSB system, the digital data signal is divided into segments each having a duration of 77.3 xcexcs and each segment is divided into 832 symbol intervals, each occupying about 0.09 xcexcs. Four of the symbol intervals in each segment are used for segment sync.
FIG. 1 is a simplified schematic block diagram illustrating the transmitter for a trellis-coded 8VSB system. As shown in FIG. 1, digital video, audio and ancillary data are combined and processed by a processor 10 in the manner described in the article cited above and in other documents describing the ATSC digital television standard and the resulting data stream is supplied to a trellis encoder 14, which generates a baseband output signal having one of eight levels during each symbol interval. Thus, three bits of data can be represented by a single symbol. A multiplexer 18 multiplexes the data symbols with segment sync and frame sync and provides a composite baseband data signal. The composite baseband signal is passed through an I-channel root raised cosine filter 20I and a Q-channel root raised cosine filter 20Q to a quadrature modulator 22 which uses the I and Q channel baseband data signals to modulate two carrier signals at the same frequency (typically 44 MHz) and in phase quadrature. The quadrature modulator 22 includes a summing element which adds the two modulated carrier signals together to form the 8VSB signal. The 8VSB signal is supplied to an RF upconverter 26 which translates the 8VSB signal to the desired RF channel. The RF signal drives an antenna 30.
At the receiver, the received RF signal is downconverted to an intermediate frequency and the IF signal is digitized and demodulated. Since the transmitted signal is a vestigial sideband signal and the baseband signal is complex, demodulation provides separation of in-phase (I) and quadrature (Q) components. Data is recovered from the I component by equalizing and slicing at eight levels. The recovered data is processed to separate video, audio and ancillary data.
Data cannot be directly recovered from the Q component of the demodulated signal. However, useful information regarding the propagation path of the data can be attained from the Q component by equalizing the Q component in conjunction with equalizing the I component and plotting the I and Q values for each symbol in a segment along the axes of a rectangular Cartesian coordinate system, so that each symbol is represented by a single dot or landing point. The landing points are typically displayed against a graticule composed of eight straight lines extending parallel to the Q axis and corresponding to the eight data levels respectively. The display of landing points is referred to as a constellation display.
FIG. 2 shows the traditional constellation for an 8VSB data segment with the Q axis vertical and the I axis horizontal. The eight straight lines of the graticule are vertical and are labelled +/xe2x88x921, +/xe2x88x923, +/xe2x88x925 and +/xe2x88x927 respectively. Ideally, each landing point would be on one of eight vertical lines. In practice, however, due to impairments in the path from the modulator to the test point, the landing points do not all lie on the vertical lines but are, on the contrary, distributed along the eight lines in bands or elongated clouds, as shown in FIG. 2. The distribution of the landing points can be used to diagnose problems in, for example, the transmitter. However, interpretation of the constellation diagram has hitherto been rather subjective because it can be difficult to discern patterns in the clouds of landing points.
In accordance with a first aspect of the present invention there is provided a method of generating a constellation display of symbol landing points along I and Q axes in a complex plane, wherein the landing points are in a band extending along a line parallel to the Q axis, by processing the coordinates of the landing points to generate a curve that optimally fits the landing points under a selected criterion, and displaying the curve in the complex plane.
In accordance with a second aspect of the present invention there is provided a modulation analyzer, for generating a constellation display of symbol landing points along I and Q axes in a complex plane, wherein the landing points are distributed along a line parallel to the Q axis, having a processor which processes the coordinates of the landing points to generate a curve that optimally fits the landing points under a selected criterion, and a display device which displays the curve in the complex plane.
In accordance with a third aspect of the present invention there is provided apparatus for testing a vestigial sideband digital transmission system employing eight-level amplitude modulation, having a front end for deriving I and Q values for symbol landing points in a complex plane, wherein the landing points are in eight elongated bands distributed along respective lines parallel to the Q axis, a smoothing processor which processes the I and Q values for the landing points to generate eight curves that optimally fit the landing points in the eight bands respectively under a selected criterion, and a display device which displays the curves in the complex plane.